The present invention relates generally to the field of industrial and utility furnaces and boilers and in particular to new and useful over fire air (OFA) port configurations for a pulverized coal-fired furnace or boiler which effectively reduce NOx production.
NOx is an unintended byproduct from the combustion of fossil fuels, such as coal. Many industrial furnaces and boilers burn pulverized coal as a primary fuel. NOx emissions have been discovered to have a negative effect on the environment, and so they are now regulated substantially throughout the world.
Most NOx in furnaces and boilers burning pulverized coal is formed during combustion from the fossil fuel. This portion of NOx formation is called fuel NOx. Fuel NOx is formed by oxidation of fuel-bound nitrogen during devolatilization and char burnout.
An effective method of reducing NOx production which has been known for many years is to reduce oxygen availability during the critical step of devolatilization. Oxygen availability can be reduced during devolatilization by removing a portion of the combustion air from the burners and introducing the air elsewhere in the furnace. This method is commonly referred to as air staging.
Over fire air (OFA) ports are typically used as part of such air staging systems in furnaces and boilers. The use of such OFA ports is disclosed, for example, in U.S. Pat. Nos. 3,048,131, 5,205,226 and 5,809,913. For a better understanding of such OFA systems, the reader is referred to Steam/its generation and use, 40th Ed., Stultz & Kitto, Eds., Copyright© 1992 The Babcock & Wilcox Company, the text of which is hereby incorporated by reference as though fully set forth herein, and particularly to Chapter 13, pp. 13-6 to 13-11.
The effectiveness of over fire air in NOx suppression depends on the quantity of over fire air, the point in the burner flame where the over fire air is reintroduced, and the rate of reintroduction. Increasing the over fire air quantity tends to lower NOx levels from the burners, but continual increase of over fire air quantity will eventually cause NOx to increase as well. This results from combustion being displaced to a region of the furnace or boiler beyond the OFA ports.
The point at which over fire air is introduced into the furnace is critical as well, since the purpose of OFA systems is to enable the chemistry to proceed through a region of lower oxygen concentration in order to suppress NOx formation as hydrocarbons preferentially scavenge oxygen. Prematurely adding over fire air will negate the benefit as the desired chemistry is disrupted. And, the rate at which OFA is added is also important, so as to avoid creating oxygen-rich regions within the furnace. It is usual to gradually introduce over fire air to the combustion process to complete combustion without locally flooding the flames with oxygen. At the same time the OFA ports must be designed with sufficient jet momentum to penetrate and supply over fire air throughout the furnace enclosure.
FIGS. 1 and 2 illustrate a common prior art arrangement of burners and OFA ports and the resulting flame paths. The furnace enclosure 10 has three levels of burners 12, 14, 16. The enclosure 10 illustrated is typical of opposed-fired boilers; that is, burners 12, 14, 16 are oriented through both the front and rear walls 30, 32 of the enclosure 10, opposite each other. The uppermost level of openings through each of the front and rear walls 30, 32 of the enclosure 10 is comprised of OFA ports 20.
In FIG. 2, the approximate flame paths 13, 15, 17 generated by each row of burners 12, 14, 16 on the front and rear walls 30, 32 are displayed. Bottom burners 12 fire horizontally, and so flame paths 13 from the opposed burners 12 collide in about the center of the enclosure 10. Unburned combustibles and hot gases flow upwardly in a path 13a concentrated in the middle of the enclosure 10. Second level burners 14 are affected by the upward flow 13a of gases and combustibles, so that second level flame paths 15 from the opposed burners 14 bend upwardly near the middle of the enclosure 10. Third level burners 16 are even more affected by the upflow of gases 13a, and so the third level flame paths 17 from these burners bend upwardly even more quickly than second level flame paths 15.
As shown, the OFA air path 22 intersects the second and third level burner flame paths 15, 17 and approaches the upwardly flowing gases and combustibles 13a. This conventional OFA port configuration of FIGS. 1 and 2, while useful, provides greatly varying effects when OFA is injected into the enclosure 10. The effect on reduction of NOx is not consistent due to differences in residence time between the burners and the OFA ports, and differences in gas flow through the furnace resulting in different interactions of the OFA and flame paths, among other factors.
The OFA configuration illustrated in FIGS. 1 and 2, when used in a 600 MW utility boiler or furnace unit for example, will have a calculated bulk flow residence time from burners to OFA ports of 2.7 seconds for the bottom burners 12, 1.3 seconds for the second level burners 14 and only 0.6 seconds for the third level burners 16. Thus, the level 3 burners 16 suffer from insufficient residence time relative to the region of introduction of OFA, which tends to raise the level of NOx produced. Often, the most efficient method of reducing NOx emissions in this type of furnace is to disable the third level burners 16.
An alternative for increasing residence time for the second and third level burners 14, 16 is to increase the distance between the OFA ports 20 and the third level burners 16. However, this also requires additional space in the upper furnace region of the enclosure 10. Thus, increasing the OFA port 20 spacing requires a taller furnace enclosure 10, thereby increasing the costs and making a bigger building.
An OFA configuration which provides consistent minimum residence time between burner and OFA port but does not require a larger furnace or disabling existing burners is desirable. Further, an OFA port air flow which is better managed for each burner level is also desirable for reducing NOx emissions.